9mm is second to .22 in a easy caliber to teach new shooters.
you have a huge number of choices in both frame size, manufacturer, type of material of pistol design and in ammo selection.
there is literally a 9mm pistol that virtually an shooter can find that will fit their hand and perform reliably without breaking the bank.
If the shooter does their job, the round will do its job.
Also, I find the 9mm a very forgiving round for people who don't get to the range as often as they should. Probably due to less flinching/recoil anticipation.